Conventionally, methods comprising polymerizing a crosslinkable monomer in situ in a column with a poor solvent as a porogen (pore-forming agent) have been predominantly used for preparing a polymer-based monolithic separation medium for liquid chromatography. For example, Patent Literature 1 describes a process for producing a liquid chromatography column containing a polymer having pores having a diameter of less than 200 nm and pores having a diameter of not less than 600 nm, which comprises reacting a polymerization mixture containing a hydrophobic vinyl monomer, a radical initiator, and a poor solvent for the vinyl monomer as a porogen, and then, removing the porogen.
However, polymer molecules evolved from a monomer tend to be aggregated in a poor solvent, since the effect of van der Waals forces between the polymer molecules is greater than the steric hindrance effect. In such a system, the formation of monolithic medium progresses through the following steps: (1) the nuclear formation by the entanglement of polymer chains evolved from a monomer; (2) the drastic increase of surface energy of the system by increasing the amount of dispersed fine microgel particles; (3) the decrease of surface energy and the phase separation by the agglomeration of microgel particles; and (4) the emergence of rapidly coarsened particle-agglomerated structure. Thus, the phase separation of the polymer phase and the poor solvent phase (porogen) extremely rapidly progresses. In such a system, a monolith comprising a nonuniform macroporous coarse particle aggregate is obtained. There are some problems with such an aggregate that the separation efficiency becomes lower, because the aggregate has a labyrinth of macro through-pore channels, and the eddy diffusion of solute becomes higher; the aggregate is mechanically fragile, and the morphological stability thereof is as low as a change in porosity due to a compressive effect is caused when a column filled with the aggregate is subjected to a high back pressure; the separation capacity of solute becomes lower, because the aggregate has a small amount of mesopores having a nanometer diameter and a extremely low specific surface area of pores; and the like.
In the meantime, as a result of a rapid progress in recent research on the phase separation of polymer solutions, it has been found that the viscoelasticity of polymer solutions greatly affects the phase separation speed (Non-patent Literature 1). In general, when a solution of low molecular weight substance is subjected to a rapid temperature change (rapid cooling or rapid heating) into an unstable area where two phases are coexisted, the spinodal decomposition is rapidly caused. As a result, eventually, the system is dominated by a surface tension and the solution is separated into two phases in sea-island structure composed of a dispersion phase containing the solute (small amount component) and a matrix phase containing the solvent (large amount component). However, in the case of polymer solutions, when there is a great difference in molecular dynamics (i.e., mobility) between a solvent molecule and a polymer, a viscoelasticity and a stress having a long relaxation time are caused because of a rapid gelation of the polymer rich phase due to the desolvation. As a result, the diffusion of the solvent molecules from the polymer rich phase is temporarily inhibited. Thus, a conventional two phase separation of the system is not caused immediately by a rapid temperature change, but a transitional three-dimensional continuous network structure having a long relaxation time is formed in the polymer rich phase. For example, in a system containing a polystyrene having a glass transition temperature of 100° C. as a small amount component and a polyvinyl ether having a glass transition temperature of −23° C. as a large amount component, wherein the component molecules have a different mobility, the formation of a transitional three-dimensional continuous network structure of polystyrene phase is observed in the course of the phase separation when a temperature of the system is rapidly changed from a room temperature to 143° C. (Non-patent Literature 2).
Patent Literature 1: JP-B-3168006
Non-patent Literature 1: Kobunshi, Vol. 52, No. 8, page 572 (2003)
Non-patent Literature 2: Physical Review Letters, Vol. 76, page 787 (1996)